Update PLAYER_ACTIVITY.md

[SkuliAdams]

Is your feature request related to a problem? Please describe.

Since #37450 introduced activity_actor as the preferred method to create activities, PLAYER_ACTIVITY.md has been out of date, as it describes how to add new activities using the old method.

Describe the solution you'd like

Update PLAYER_ACTIVITY.md to reflect the new activity_actor system.

Describe alternatives you've considered

Not documenting the changes in the /doc/ folder.

[Night-Pryanik]

Closed by #39636.